1. Field of the Invention
The present invention relates to a radio filter employing transmission lines, and more particularly a radio filter of combline structure with a capacitor compensation circuit connecting multiple filter layers through via-holes.
2. Description of the Related Art
It is generally desired to manufacture a portable radio communications system, like a mobile phone, being of small size and low cost. Of course, this applies to other kinds of equipment and therefore development of various technologies is required.
In order to reduce the size of the portable radio communications system having a radio filter for receiving or transmitting signals of a desired frequency band only, while blocking other noise signals, transmission lines (stripline or micro stripline) are used which occupy much less space than passive elements.
Such a strip-line filter is disclosed in U.S. Pat. No. 4,963,843 granted to Motorola on Oct. 16, 1990, in reference to which the conventional combline stripline filter is briefly described as follows:
The conventional combline stripline filter comprises a substrate having top and bottom surfaces each with a conductive material thereon forming a respective ground plane. The substrate also has an inner circuitry layer, which includes a ground area consisting of a conductive material having a plurality of angled edges, and coupled to at least one of the ground planes, and at least two combline resonators. Each resonator comprises a strip of conductive material. The strips are substantially parallel to one another. Each strip has adjacent first ends coupled to at least one of the ground planes, and adjacent second ends capacitively coupled to the ground area. Each of the adjacent second ends further has at least one angled edge disposed opposite a corresponding angled edge of the ground area. Each strip further has a respective extension portion at the second ends disposed substantially at right angles to the parallel strips. Each extension portion has a respective edge disposed opposite and capacitively coupled to the ground area. One of the extension portions is coupled to the radio signal.
However, such a conventional stripline filter employing the pattern capacitor as described above suffers from increased layout size and increased error of the pattern capacitor caused by electrical interference. In addition, it is hard to connect with other devices, and its capacitance cannot be accurately measured because of its being capacitively coupled to ground.
Furthermore, its manufacture is complicated because the type of material used for the substrate determines the capacitance with respect to the ground area. Moreover, since the connections with the input pads, output pads, and ground are made at the ends of the substrate, the sizes and positions of other devices connected thereto are very limited.
It is an object of the present invention to provide a radio filter of combline structure with a capacitor compensation circuit connecting multiple layers through via-holes.
It is another object of the present invention to provide a radio filter of combline structure with a capacitor compensation circuit employing the capacitor of lumped element as the capacitor compensator.
It is still another object of the present invention to provide a radio filter of combline structure with a capacitor compensation circuit, which can be embodied on an ordinary substrate.
According to the present invention, a radio-filter of combline structure with a capacitor compensation circuit, comprises a transmission line filter having at least a pair of transmission lines arranged between input and output terminals for filtering the input signals through the input terminal to select signals of a given frequency band delivered to the output terminal. Each of the transmission lines has a via-hole at each of its ends, a capacitor compensator of lumped element connected through one of the via-holes to one of the transmission lines for providing capacitance between the transmission line and ground, and a ground layer connected to the other via-hole which is not connected to the transmission line in order to ground the transmission line.
The present invention will now be described more specifically with reference to the attached drawings which are shown for exemplary purposes only.